You know, the best ones aren't always the most technical ones (and I say that as someone with many many years development experience) - I would ask your friends and community for recommendations, rather than trying to figure it out yourself.

I would also look for ones who are investing heavily in one community. Someone who does exclusively Python in London, sponsors all the conferences, and has done for a while is likely to know which places are nice to work at, where pays well comparatively, and so on.

trust your gut. they're sales people, not technical people, so judge them by the appropriate metrics. i.e.:

are they putting you in front of the right companies? are they asking you questions about your needs? are they pitching you on the job (it should be worthy of a pitch), as well as vice versa? are they working with the right hiring budget?

these are the basic questions you use to judge the quality of a sales person. they have to "get it" on a relationship level, not a technical level.